crtime: add (Mono).add and ClockReading.

This may make it a bit easier to adopt Mono in libraries without forcing
us to immediately update all callers, by making the library generic over
crtime.ClockReading, which is implemented by both time.Time and
crtime.Mono.

Author: stevendanna

crtime/monotonic.go

@@ -27,6 +27,14 @@ import (
 // used.
 type Mono time.Duration
 
+type ClockReading[T any] interface {
+	// Add returns a ClockReading time.Duration in the future from a given
+	// ClockReading.
+	Add(time.Duration) T
+	// Sub returns the duration elapsed between two clock readings.
+	Sub(T) time.Duration
+}
+
 // NowMono returns a moment in time in terms of a monotonic clock. It is faster
 // than time.Now which also consults the wall clock.
 func NowMono() Mono {
@@ -39,6 +47,11 @@ func (m Mono) Sub(other Mono) time.Duration {
 	return time.Duration(m - other)
 }
 
+// Add returns the time.Mono m+d.
+func (m Mono) Add(d time.Duration) Mono {
+	return Mono(time.Duration(m) + d)
+}
+
 // Elapsed returns the duration that elapsed since m.
 func (m Mono) Elapsed() time.Duration {
 	return time.Duration(NowMono() - m)